1. Field of the Invention
This invention relates to Read Only Memory (ROM) devices, and more particularly to improved structural arrangements thereof, and to methods of manufacture thereof.
2. Description of Related Art
ROM devices are standard components of modern computer systems. A ROM comprises an array of Metal Oxide Semiconductor Field Effect Transistor (MOSFET's) arranged in columns and rows, wherein predetermined MOSFET's are either permanently conductive or nonconductive as a function of the variety of transistor. The alternative on/off operation with changes in the states of operation of the MOSFETs is adapted to use for storage of data, which remains in the device when the external power supply is off.
A ROM device includes an array of parallel, closely spaced lines regions formed of a heavily doped impurity in a semiconductor substrate having an opposite type of background impurity. On the surface of the substrate an insulating layer is formed thereon. Another array of closely spaced conductive lines formed on the surface of the insulating layer is arranged at right angles to the spaced lines in the substrate. Insulating layers are formed on the upper array of conductive lines. A metallurgy layer connects the two arrays of lines to circuits to address the lines and to read the data stored in the ROM, as is well known in the art.
At the intersection of a conductive line in the upper array which is commonly referred to in most cases as a "word line" and a pair of adjacent lines in the substrate, known in most cases as the "bit lines", a MOSFET is formed. The spaced lines in the substrate comprise the source and drain of the MOSFET. The conductive word line serves as the gate electrode of the MOSFET. Certain predetermined MOSFET's can be made permanently non-conductive (turned off).
A problem with the trend in the semiconductor industry is the ever increasing density of circuits on a device. Accordingly, an object of this invention is to provide an improved design for a very high density ROM device.
Another objective ever desirable in electronic devices and other technologies is the simplification of the structure and operation of devices. In the case of the very large number of circuits in the state of the art ROMs the need for simplification of structure and operation is as desirable as ever. Accordingly, it is an important object of this invention is to provide for simplification of the structure and the operation of ROM devices.